You see, insurance companies don't like this idea of offering up your car to someone else for dough. In fact, they can legally deny coverage of your vehicle if you do so. Current laws In California, and many other states, prohibit the private rental of your vehicle to somebody else, but that could change soon. A bill (AB 1871) written up by Assemblyman Dave Jones (D-Sacramento) aims to change the state's car insurance laws to allow personal vehicles to be rented out without the risk of losing insurance.
You may be wondering why anyone would want to rent out their ride to a stranger. It makes sense if you look at a heavily congested city like San Francisco (pictured). Many residents of the Bay Area do not own personal vehicles and must rent one for trips outside of the city limits. On the flip side, people who do own vehicles rarely use them. In situations like this, private car sharing could be a perfect answer, if the law allowed it.
[Source: SFGate via Green Car Reports | Image: http2007 - C.C. License 2.0]